Concrete slab edge interior insulation length extension

[Julio Fernández Amodia]

Hi to all, 

I have been going in circles about something for a while already and I would like to share my conclusions about it. The issue is the following: I have a series of concrete slabs that stick out from the facade, this is a design imposition given by the architect. I have to estimate how much insulation should extend to the interior, that is the length (distance in the attached drawing). To my understanding, the only method to address this is to calculate for different lengths the PSI value and draw a curve to see when the impact starts to stabilize. Is there any other method or recommendation about how to address this? 

Thanks

[archi...@gmail.com]

Hi.

I think that your system is correct to know when the psi value cannot be further decreased. (By the way which dimension system are you using? Gross external measures or net internal measures?)

BUT to determine if your detail is correct I would not concentrate on the psi-value, rather on its internal surface temperature or better on its fRsi value.

Ciao

Fabrizio

[Julio Fernández Amodia]

Hi Fabrizio,

Thanks for your response. I am using external dimensions for the psi values calculations.

I thought about the internal surface temperature and fRsi, but it was not making much of an impact, since the outdoor temperatures during the coldest month are somehow mild, only 25cm of insulation extension was required to meet the requirements. Therefore, I though that a higher extension based on the PSI values was more representative of the insulation effectiveness.

Cheers,

Julio

[archi...@gmail.com]

Hi Julio

The only value that correctly and unambiguously represents the behaviour of a thermal bridge regardless of the outside temperature (Te) is precisely the factor fRsi = (Tsi - Te) / (Ti - Te)

In accordance with EN ISO 13788 by verifying that the fRsi value of the thermal bridge is greater than the fRsm factor (temperature factor for mould risk of the most critical month, typically the coldest), it is certain that mould cannot form, let alone surface condensation at the thermal bridge. Generally with TB's fRsi value  > 0.75 or 0.85 the TB can be considered correct.

The Psi value does NOT characterize the TB, it is only a calculation value that compensates for underestimation or overestimation of the 1D calculation of heat loss.

There are certain TBs, such as cantilevered balconies, that no matter how properly insulated they are, they can never have Psi values less than 0.25-0.30 W/mK. But the important thing is that if, as mentioned, they are properly insulated, they have very high fRsi values, even 0.90 or 0.95, and this confirms to us that they are correct.

From what you write about the check you made with the fRsi value in your case "only" 25 cm of insulation lapel would actually be sufficient.

Ciao

Fabrizio

[archi...@gmail.com]

Hi again Julio.

I would definitely check fRsi in the red points I added on your images.

The harder to positively verify are certainly those on the window frames.

Ciao

Fabrizio

[Reichard, Georg]

Just to set the record straight, since there may be many (students) following this thread using THERM for a variety of reasons: I would like to respectfully reject the bold notion that the Psi value does NOT characterize a thermal bridge.

It doesn't describe the condensation risk of the TB, but it very much describes the severity in thermal losses of the same. This is a critical performance characteristic for interface details, when a design team strives for high-performance building enclosures, such as in net-zero or passive house designs.

With highly insulated wall systems, thermal bridges around interfaces, e.g around windows, increase the contributions of TBs  (percentage wise) more significant than in "standard" constructions. Of course, careful design can again mitigate these impacts, which is directly related to the evaluation and comparison of Psi values across these details.

So just because Psi values are not the right means to evaluate the surface condensation risk as Fabrizio points out correctly, it does not mean that a Psi value doesn't characterize a thermal bridge in terms of thermal performance.

Cheers,

Georg

 

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From: lbnl-...@googlegroups.com <lbnl-...@googlegroups.com> on behalf of archi...@gmail.com <archi...@gmail.com>
Sent: Wednesday, June 19, 2024 11:26 AM
To: THERM <lbnl-...@googlegroups.com>
Subject: [LBNL-THERM] Re: Concrete slab edge interior insulation length extension

 

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[David Collins]

Yes, Georg,  I'd have to agree with you.  PHI uses both Psi values and fRsi values to characterize TBs.  The former for heat loss and the latter for comfort.

- David